Control of insect pests by chemical means has long been a useful method to protect crops from damage caused by insect-attack and infestation. More recently, methods to control insect crop damage have been introduced which are specific to the target insect and avoid environmental and ecological compromise associated with traditional pesticide usage. One of these methods employs a genetically modified crop which produces insect-specific toxins, e.g., the Cry toxin from Bacillus thuringiensis. However, the B. thuringiensis-Cry-toxin-expressing crop may exhibit varying degrees of protection from an array of lepidopteran pest species. For example, CryIA(c)-expressing cotton varieties are highly resistant to tobacco budworm, Heliothis virescens, but only moderately resistant to cotton bollworm Helicoverpa zea (J. H. Benedict et al., 1996, Journal of Economic Entomology, Vol. 89(1), p. 230).
Another such method of insect control is the application of biological agents such as a nucleopolyhedrosis virus (NPV)(U.S. Pat. No. 4,668,511), or recombinant nucleopolyhedrosis virus (rNPV) (U.S. Pat. No. 5,662,897 and U.S. Pat. No. 5,858,353). However, NPV and rNPV may vary in the level of virulence/potency against various insect species, depending upon the host range of the viral vectoring agent and the potency of the toxin encoded by the inserted gene. For example, the insect species Helicoverpa zea is highly susceptible to the NPV and rNPV designated HZNPV and HzAaIT, respectively, but only moderately susceptible to the Autographa californica NPV (AcNPV) or its rNVP, AcAaIT (Treacy et al., 1999, Proceedings Beltwide Cotton Conf., pp. 1076-1083). Although the combination of applying a recombinant nucleopolyhedrosis virus which contains a vector which is moderately virulent to the target insect species to a transgenic crop line has been described, (All and Treacy, 1997, Proceedings Beltwide Cotton Conf. p. 1294), neither the transgenic crop nor the rNPV agent, alone or in combination, provided the level of insect control needed to prevent crop loss on a commercial basis.
Therefore, it is an object of this invention to provide a method of synergistic insect control useful for preventing crop damage and economic loss caused thereby.
It is another object of this invention to provide a method for the enhanced protection of a transgenic crop from the devastation and damage caused by insect attack and infestation.
It is a feature of this invention that the synergistic insect control and crop protection methods provided are specific to the target insect species and demonstrate enhanced environmental and ecological compatability, while providing commercially acceptable levels of insect control and crop protection.
Other objects and features of the invention will be apparent to those skilled in the art from the following description and the appended claims.
The present invention provides a method for synergistic insect control which comprises applying to the locus of a transgenic crop a synergistically effective amount of a recombinant insect virus containing a vector which is highly virulent to said insect.
Further provided is a method for the enhanced protection of a transgenic crop from damage caused by insect attack and infestation.
Although chemical pest control has been an effective means of controlling important agronomic insect pests, more target insect-specific methods of control have been introduced. Among these insect-specific methods are the use of a transgenic crop which has been genetically altered to produce an insect toxin such as Bacillus thuringiensis (Bt) or the use of a naturally occurring virus such as the nucleopolyhedrosis virus (NPV) or recombinant NPV (rNPV). However, the transgenic crop which produces a Bt toxin may exhibit a less than satisfactory degree of protection from the targeted insect. Similarly, naturally occurring and recombinant insect viruses often demonstrate varying degrees of efficacy when used as the sole method of insect control.
Although the use of a combination of an rNPV which contains a vector which is moderately virulent to the target insect species and a transgenic crop has been described, the results achieved were not satisfactory for commercial insect control when said rNPV was applied alone or when said rNPV was applied in combination with a transgenic crop genetically altered to produce an insect toxin.
Surprisingly, it has now been found that the application of a recombinant insect virus which contains a vector which is highly virulent to the target insect species to a transgenic crop, preferably a transgenic crop which has been genetically altered to produce an insect toxin (insecticide), demonstrates a significant synergistic effect (i.e. the resultant insect control is much greater than that which could be predicted from the insect control of the virulent recombinant insect virus when used alone or from the insect control of the transgenic crop when used alone). This synergistic effect enables a commercially useful level of insect control via a non-chemical biological means. Further, the synergistic insect control method of the invention allows for effective resistance management compatable with sustainable agriculture practices which are environmentally and ecologically sound.
In accordance with the method of the invention, the application of a synergistically effective amount of a recombinant insect virus, preferably a recombinant nucleopolyhedrosis virus (rNPV), containing a vector which is highly virulent to the target insect species to a transgenic crop variety, preferably a transgenic crop which is genetically altered to produce an insect toxin, provides synergistic control of the insect pest. That is, the application of the virulent recombinant insect virus to the transgenic crop results in a combination of insecticidal components which produces a greater insecticidal effect than that which would be expected from the individual insecticidal components employed individually (synergistic effect).
Recombinant insect viruses containing a highly virulent vector which are suitable for use in the method of invention include rNPVs such as HzNPV, HzAaIT, EGTdel, or a combination thereof. Transgenic crops which produce an insect toxin suitable for use in the method of invention include Bt expressing lines of maize and cotton(BTK lines), such as NuCotn 33B(trademark), a transgenic cotton variety derived from Deltapine DP5415(trademark) by the Bollgard(trademark) transformation event, or transgenic maize varieties such as those which express the MON 810(trademark) transformation event (YieldGard(trademark), Monsanto Co.).
In actual practice, the virulent recombinant insect virus may be applied in the form of a formulated composition, such as a wettable powder, to the locus, foliage or stems, preferably the foliage, of a transgenic crop, particularly a transgenic crop which has been genetically altered to produce an insect toxin. A preferred formulation is that described in co-pending U.S. patent application Ser. No. 09/094,279, filed Jun. 9, 1998, incorporated herein by reference thereto.
The synergistically effective amount of the virulent recombinant insect virus may vary according to prevailing conditions such as the degree of insect resistance of the transgenic crop, the application timing, the weather conditions, the mode of application, the density of the insect population, the target crop species, the target insect species, and the like. In general, synergistic insect control may be obtained when the virulent recombinant insect virus is applied to the transgenic crop at rates of about 1xc3x971010 occlusion bodies per hectare (OB/ha) to about 1xc3x971013 OB/ha, preferably about 5xc3x971010 OB/ha to about 12xc3x971011 OB/ha.
In order to facilitate a further understanding of the invention, the following examples are presented primarily for the purpose of illustrating more specific details thereof. The invention should not be deemed limited thereby except as defined in the claims.
In the following examples, synergism for two-way insecticidal combinations is determined by the Colby method (Colby, S. R., Weeds, 1967 (15), pp.20-22), i.e. the expected (or predicted) results (percentage of insects eliminated) of the combination is calculated by taking the sum of the results for each insecticide component applied alone and subtracting the product of these two results divided by 100. This is illustrated mathematically below, wherein a two-way combination is composed of component X plus component Y.             (              X        +        Y            )        -                  X        ⁢                  xe2x80x83                ⁢        Y            100        =      Expected    ⁢          xe2x80x83        ⁢    results  
If the actual observed results are greater than the expected results calcualted from the formula, synergy exists.
In the present invention, the percent insect control (no external insecticide applied) exhibited by a transgenic crop of this invention relative to a closely related control crop could be represented by X; and the percent control of a recombinant insect virus of the invention when used on the control crop could be represented by Y. The foregoing Colby formula can be used to calculate the expected percent control for the combination of the virus and the transgenic crop. If the observed results (actual percent control) of the combination of the transgenic crop treated with the virus is greater than the calculated expected results, then the combination is synergistic.